Gamma-Ray Output Spectra from - T-2 : LANL

Operated by the Los Alamos National Security, LLC for the DOE/NNSA

Gamma-Ray Output Spectra from 239Pu Fission

J.L. Ullmann (For the DANCE collaboration)

LANSCE-NS Los Alamos National Laboratory Los Alamos, New Mexico USA

Workshop on Fission Experiments and Theoretical Advances (FIESTA)

Sept. 10 - 12, 2014 Santa Fe, New Mexico

LA-UR-14-27044


Collaborators and Acknowledgements

Slide 1

S. Mosby, M. Jandel, T.A. Bredeweg, A. Couture, R.C. Haight, J.M. O’Donnell, D.J. Vieira, J.B. Wilhelmy,

A. Hayes-Sterbenz, P. Talou, I. Stetcu, T. Kawano Los Alamos National Laboratory

C.-Y. Wu, A. Chyzh, J.A. Becker, J. Gostic, R. Henderson, E. Kwan Lawrence Livermore National Laboratory

Support provided by

US DOE / NNSA Contract DE-AC52-06NA25396

(Los Alamos National Security, LLC) Contract DE-AC52—07-NA27344

(Lawrence Livermore National Security, LLC) American Reinvestment and Recovery Act


Fission Physics

Slide 2

Initial Excitation

Sn

En

Eγ

239Pu(n,f) νn = 2.9 νγ = 7.2 (Thermal)

Initial fragment – high spin, excitation Neutron decay – removes excitation Gamma decay – (E1) removes spin and energy Gammas – from fragment decay

- neutron rich (? Structure) - Fragment mass distribution changes with neutron energy


Gamma-ray output from 239Pu(n,fission)

•  Prompt gamma ray emission from fission not well studied –  Only 1 published spectrum for 239Pu(n,f) – at thermal –  (V.V. Verbinski, Phys. Rev. C 7, 1173 (1973) ) –  Other measurements – but do cover wide gamma energy range

•  Experiment at LANSCE moderated white neutron source –  Need fission tagging – use LLNL/LANL PPAC –  Gammas detected using Detector for Advanced Neutron Capture

Experiments (DANCE) “4π” detector –  Gammas ± 20 ns from fission event –  Direct measurement of multiplicity and total energy distribution

Slide 3


DANCE gamma-ray calorimeter

Slide 4

•  160 BaF2 crystals – each 0.75 liter •  Inner radius = 17 cm,

crystal depth = 15 cm •  6LiH inner sphere to absorb

scattered neutrons •  Internal conversion plus

absorption in LiH may affect low-energy gamma spectrum

(20 spills (To)/sec)

Neutron Flux at Monitor

Operated by the Los Alamos National Security, LLC for the DOE/NNSA Slide 5

DANCE and LANSCE

DANCE ball (Open)

6LiH sphere in center

(Los Alamos Neutron Science Center (LANSCE)


Fission tagging using PPAC

Aluminized Mylar

(1.4 μm):

239Pu deposit (463 μg/side)

3 μm Ti foil

Rubber Cement

Polyimide Ring

Cu ring

Spring retaining ring

0.125 in 0.125 in Cathode Anode Anode

PPAC Params Gas = Isobutane

P = 4.5 Torr Flow = 4 cc/m

239Pu (total) = 2.43 mg/cm2

(0.7 cm dia deposit) 99.967% enriched

PPAC Target Assembly

LLNL / LANL / MSI PPAC 4.37 cm dia X 4.77 cm long

Fission efficiency ~ 70%


DANCE response correction crucial

Single gamma efficiency ε = 0.85 7 gammas => ε = (0.85)7 = 0.40

Methods of response correction “Spectrum stripping” – calc response, subtract starting at

highest energy (eg. R. Billnert, et al., Phys Rev C 87, 024601 (2013) “Inverse Methods” – solve O = R I for input spectrum I

1-dimensional or 2-dimensional O and I eg. A. Chyzh, et al., Phys. Rev. C 90, 014602 (2014) 1D: Eγ, Mult each unfolded 2D: Unfold Etot vs Mult Matrix

“Forward Methods” – Assume spectra, simulate response and compare Iterate spectra until fit Ultimate – use a real physics model with parameters Experimental approach – don’t depend on physics model Parameterize data analytically NOT (!!) a physics model (but may be motivated by physics)


Detector Response Correction (Preliminary results shown in this figure!)

ε = 0.85 (Geometric) ε = (0.85)7 for 7 gamma rays = 0.40

Slide 8


Parameterized fission spectra

Multiplicity •  Sum over two distributions

Mγ = M1 + M2

•  Assume Multiplicity ~ Spin distributions P(J) ~ (2J+1)e-J(J+1)/B^2

(Wilhelmy, Phys Rev C 5, 2041 (1972) )

Gamma energy distribution •  P(ε) ~ T(ε) ρ(ε) T(ε) ~ A ε3 (E1), ρ(ε) ~Bea(Eo-ε)

•  Best fit: P1(ε) ~ ε2e-(a1+Mγb1)ε P2(ε) ~ ε3e-(a2+Mγb2)ε (Fit params: a1, a2, b1, b2)

•  Observed Gamma spectrum is sum over many fission products •  Different excitation energies, temperatures, multipolarity

•  Parameterization – not a physics model


Procedure for fitting parameters

•  Generate event consisting of Mγ = M1 + M2 gamma rays with energy εi from P(M1,2) and P1,2(εγ)

•  Transport all gammas from event through GEANT4 model of DANCE to produce “experimental” values

•  Compare to measured values – calculate χ2

•  Mcr chosen to avoid overlapping clusters •  Mcr easily simulated by GEANT •  Etot (Tresh = 150 and 400 keV) •  Mcr (150 keV Threshold) •  Εγ (150 keV threshold) •  More weight to higher threshold

•  Vary parameters at random to find minimum χ2

Iterated “Simulated Annealing” technique Vary parameters over range 1±δ P(Δ) = e- Δ/T

Slide 10


DANCE response to fission-spectrum neutrons

Total Efficiency for neutrons 1.3%

Detour: Fission neutron response

Slide 11

Cum

ulat

ive

fract

ion


239Pu(n,fission) Cross Section

Current results gated on 10.93 + 11.89 eV 1+ resonances Thermal, 7.82, 22.26, 75.0 1+ and 32.33 0+ resonances have similar spectra


Best-fit parameters

Slide 13

( ) 21/)11(112)( 11BMMeMMP +−+=

εγ

γεε )1(21

1)( bMaeP +−=

)()()( 21 MPMPMP +=γ

B1 = 6.66 ± 0.15 B2 = 2.54 ± 0.12 a1 = 3.80 ± 0.12 a2 = 1.53 ± 0.14 b1 = 0.0428 ± 0.0019 b2 = 0.0522 ± 0.0034

( ) 22/)12(212)( 22BMMeMMP +−+=

εγ

γεε )2(32

2)( bMaeP +−=


Raw Data vs Parametrized fit

Slide 14

(All spectra for 150 keV Ecr threshold)

Ecrystal Total gamma-ray energy

Crystal Multiplicity


Theoretical Calculations

Slide 15

Monte-Carlo Hauser-Feshbach Model I. Stetcu, P. Talou, T. Kawano, M. Jandel, Phys. Rev. C 90, 024617 (2014) T. Kawano, P. Talou, M.B. Chadwick, T. Watanabe, J. Nucl. Sci. Tech. 47, 462 (2010) N. Becker, P. Talou, T. Kawano, Y. Danon, I. Stetcu, Phys. Rev.C 87, 014627 (2013)

•  Fragment mass distribution semi-empirical •  Radiative strength functions from RIPL-3 •  Level density was Gilbert-Cameron formalism •  Spin Distribution:

Io = ground-state moment of inertia T = fragment temperature Α = adjustable parameter


Results – Gamma-ray Multiplicity

Slide 16

239Pu(n,f) Corrected γ Multiplicity

• 150 keV Threshold • Unfolded – 1D from Chyzh,

Phys. Rev. C 87 034620 (2014) • MCHF from Stetcu, LA-UR-14-23128

(α = 1.5)

239Pu(n,f) Measured Crystal Multiplicity very sensitive to threshold

Average Raw Crystal Multiplicity Thresh <Mcl> 150 keV 7.2 300 5.7 400 4.7


Results: 239Pu(n,f) Gamma Energy

Slide 17

Measured (before response correction) cluster energy for Cluster Multiplicities 4,8,12

• Response-corrected gamma energies (all multiplicities)

• MCHF Calculation (Stetcu) • Unfolded 1D (Chyzh)

Ecluster


Results: 239Pu(n,f) Total gamma-ray energy

Slide 18

• Response-corrected gamma energies (all multiplicities)

• MCHF Calculation (Stetcu, α = 1.5)


239Pu(n,f) Average γ Multiplicity and Etot,γ

• Uncertainty in <Etot> ~ Fitting uncertainty, determined as σ of 14 iterations with lowest χ2

• <M> very sensitive to detection threshold!

<M> <Etot,γ> This Work (10.93 eV) 7.15 ± 0.09 7.46 ± 0.06 Pleasonton (thermal) 6.88 ± 0.35 6.73 ± 0.35 (Nucl. Phys. A 213, 413 (1973) )

Verbinski (thermal) 7.23 6.81 (Phys. Rev. C 7, 1173 (1973) )

MCHF (Stetcu/Talou) 7.05 6.74 (Thermal, 140 keV thresh. α=1.5) LA-UR-14-23128

Madland Summary 6.74 Chyzh, “1D” Unfolding 7.50 7.30 (Phys. Rev. C 87, 034620 (2013) ) Chyzh “2D” Unfolding 7.93 7.94 (Phys. Rev. C 90, 014602 (2014) )


Unexpected detail: (n,γf)?

Slide 20

Resonances we studied looked to be quite similar – but is that universally true? (S. Mosby – 239Pu(n,γ) analysis)

S. Mosby

(Preliminary analysis !!)

<νn> dips at several weak L=0 resonances 35.5 eV 1+ 41 1+ 44.5 1+ 52.7 1+

Neutron multiplicity measurements E. Fort, Nucl. Sci. Eng. 99, 375 (1988) Gamma multiplicity measurements Yu. Ryabov Nucl. Phys. A216, 395 (1973)


Resonance properties – Raw data

Slide 21

Gamma multiplicity Total Gamma Energy


Summary and Conclusions

•  Measurements of distribution of multiplicity, gamma energy, and total gamma energy (“forward modeling” parameterization of data) – Needs high-segmentation, 4π capability !!

•  Average multiplicity in agreement with previous measurements – sensitive to thresholds

•  Average Etot ~ 10% higher than previous

•  Theoretical modelling reproducing 239Pu data, but with adjustable parameters

•  Still some puzzles; (n,γf)?

•  “More work to be done!”

Slide 22


Extra Slides

Slide 23


Why study fission gammas?

Slide 24

•  Applications – need to know distribution of gamma-ray multiplicity, gamma energy, total gamma energy –  Reactors: heating, decay heat –  Non proliferation –  Illicit nuclear materials (portal monitors)

•  Physics of fission –  Fission products – high-spin, neutron-rich –  Decay properties constrain models


<Eγ> Changes with neutron energy •  Fragment mass distribution changes with neutron energy •  Madland formula reflects changing products and J (excitation) •  Etot = 6.741 + 0.117 Tn(MeV) -0.0002 Tn

2 MeV •  Low energies resonances – no significant change in gamma properties

(Thermal and 100 keV should have similar <Eγ> )

D.G. Madland, Nucl. Phys. A772,113 (2006)

Slide 25


PPAC Performance

PPAC Pulse Height

Pulse Height units

Slide 26


Measure Mult (νγ), Egam, Etot and Neutron energy

No fission tag (Capture + (1-ε) Fission)


•  Philosophy – experimenters want to measure something, •  Not just fit parameters to a theory •  Forward method – not a physics model

•  Parameters motivated by physics •  but really only parameters •  Represent data

Response Correction / Unfolding


Fission neutrons – Ave energy ~ 2 MeV BUT – High energy tail ! (Maxwellian)

Fission neutron effects – MCNP by TNT Transport 252Cf fission neutron spectrum into DANCE

Detour: Fission neutron response

( MCNP Calculations by T. Taddeucci )

Slide 29


Detector Response Function

Multiplicity •  Sum over two distributions

Mγ = M1 + M2

•  Spin distributions P(J) ~ (2J+1)e-J(J+1)/B^2

(Wilhelmy, Phys Rev C 5, 2041 (1972) )

•  Assume P(M) = P(J) (Number of gammas = spin) (Good for E1, M1 roughly)

•  2 fission products => P(Mγ) = P(M1) + P(M2) B’s are fitted – M’s (J’s) are random variables

Slide 30


Detector Response Function

Gamma energy distribution •  P(ε) ~ T(ε) ρ(ε)

• For E1 transitions: T(ε) ~ Aε3

• “Constant Temperature” ρ(ε) ~ BeαEx = Bea(Eo-ε)

• P(ε) ~ ε3e-aε

•  Lemaire calculation: P(ε) ~ ε2e-βε

(S. Lemaire et al., Phys. Rev. C 73, 014602 (2006))

•  Best fit: : P1(ε) ~ ε2e-(a1+Mγb1)ε P2(ε) ~ ε3e-(a2+Mγb2)ε (Fit params: a1, a2, b1, b2)

•  Observed Gamma spectrum is sum over many fission products •  Different excitation energies, temperatures, multipolarity

Details: J.L. Ullmann, et al., Phys. Rev C 87, 044607 (2013) M. Jandel et al., Los Alamos Report LA-UR-12-24975

Slide 31


Multiplicity dependence of Etot and Egam

Slide 32

Cry

stal

Mul

tiplic

ity

Cry

stal

Mul

tiplic

ity

E gamma (MeV) E total (MeV)

Egam: no strong multiplicity dependence Etot: Etot roughly proportional to multiplicity


Multiplicity sensitive to DANCE threshold

Primary Multiplicity Etot (MeV)

Multiplicity, total energy with different thresholds determined from MCHF calculation


High-energy gammas

F.S. Dietrich, J.C. Browne, W.J. O’Connell, and M.J. Kay, Phys. Rev. C 10, 795 (1974)


Changes to Analysis 29-Aug-12

• Parameterization • Method fitting: “Metropolis Algorithm” + Simulated Annealing

• Parameters fitted for Chi-square minimization • 7 spectra (P9VA2MD)

• Overestimates high-energy effect • 4 Spectra (P9VA2ME)

• Uncertainties in <E>, <M> • New method of minimization implies cannot easily use previous technique for estimating uncertainties

• Use % Std of 14 best-fit (lowest Chi2) iterations • BUT – use Value of best-fit iteration!

• Effect of threshold on measured multiplicity


F. Pleasonton, Nucl. Phys. A213, 413 (1973) (Thermal) V.V. Verbinski, H. Weber, and R.E. Sund, Phys. Rev C 7,1173 (1973) (Thermal) Other measurements – incomplete Gamma energy range

Previous Measurements

• “Unfolding” of measured spectrum critical to results

• Pleasonton also determined fission product ID from Doppler shift.

1 NaI

Slide 36


PPAC Assembly

Slide 37

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Gamma-Ray Output Spectra from - T-2 : LANL